This invention relates to a novel method of electrically processing a completely-assembled CRT (cathode-ray tube) to reduce, and preferably to completely suppress, both arcing and afterglow during and after the subsequent operation of the CRT.
A CRT comprises an evacuated envelope which includes a neck, a funnel and a faceplate opposite the neck. A luminescent viewing screen is supported on the internal surface of the faceplate. A conductive coating on the inside of the funnel is one plate of a filter capacitor, and also is the anode of the CRT. An external coating on the funnel is the other plate of the filter capacitor. A mount assembly including one or more electron guns is supported from a glass stem that is sealed into the neck. Each electron gun includes a cathode, a control electrode, a screen electrode, a focus electrode and a high-voltage electrode.
After the CRT is completely assembled, evacuated and sealed, the mount assembly is electrically processed so that the electron gun or guns become operative, their operation stabilized and their operating lives lengthened. This processing includes (i) "hot-shot" wherein the cathodes are rendered electron emitting, (ii) "low-voltage aging" wherein the electron emissions are stabilized and (iii) "spot-knocking" wherein arcing and stray electron emission from the electrodes during operation are reduced, afterglow is reduced or completely suppressed and the operation of the CRT is further stabilized.
Afterglow is the effect wherein a completed CRT, installed in a chassis and operated in a normal manner, may continue to emit light from the viewing screen after the normal operating voltages are removed from the mount assembly. This effect, which may linger for minutes or hours, is attributed to the coincidence of: (1) a residual high voltage on the anode of the CRT and on the high-voltage electrodes of the mount assembly with respect to the other electrodes of the mount assembly, and (2) sites on the electrodes of the electron gun from which electrons can be emitted when they are under the influence of the electric field produced by the residual high voltage on the high-voltage electrodes. Emitted electrons under the influence of the electric field move toward, and impinge upon, the viewing screen producing the afterglow.
Spot-knocking may be conducted before or after the hot-shot. In ordinary spot-knocking, all of the gun elements that are not connected to the anode are connected to ground potential, and positive voltages of sufficient magnitude are applied to the anode, causing arcing to occur between gun elements connected to the anode and the adjacent gun elements. The positive voltages may be varying DC voltages, such as clamped AC or rectified AC. The positive voltages may be pulses obtained by capacitive discharge through a ball gap. Where varying DC voltages are employed, RF (radio frequency) pulses may be imposed on the varying DC voltages. In every case, spot-knocking has the effect of producing arcs which burn particles and microprotrusions from the surfaces of the electrodes. If the arc currents are high enough and properly located, substantially all of the particles and protrusions will be vaporized or fused, and few free particles will be produced. If the current is not high enough and/or is not properly located, remnants will remain which are sites for stray electron emissions in the presence of electric fields. If the arc current is too high, an unacceptable amount of particles will be generated, which particles may interfere with the subsequent operation of the CRT, including being a cause of arcing and/or afterglow.
In one prior method of electrically processing a CRT, disclosed in U.S Pat. No. 4,395,242 to P. R. Liller et al. and referred to herein as SERP, the portion of the focus electrode facing the high-voltage electrode is heated at temperatures above about 700.degree. C. prior to RF spot knocking (RFSK); that is, spot-knocking the electrodes with high varying DC voltages having high-intensity RF pulses superimposed thereon. Heating is preferably achieved by bombarding the focus electrode with electrons from the cathode of the CRT. Where this prior method is applied to a tube that is receiving its initial electrical processing, the heating step is conducted during the low-voltage aging step, and the RFSK is applied after the IIP structure is mounted on the CRT, as in prior practice. Where the novel method is applied to tubes that were rejected for excessive afterglow; that is, the tubes are being reprocessed, the heating step is conducted and then is followed by the additional step of RF spot-knocking. While this prior SERP method is successful with most tubes, there is a small percentage of tubes that exhibit unacceptable afterglow even after several cycles of this prior method. Also, because of the severity of this prior method, undesirable particles may be generated which may interfere with the proper operation of many tubes.